In a small heat-proof, fry pan, on the grill, heat olive oil to medium hot; add onions and mushrooms and saute' until onions are limp. Set aside.

In a small mixing bowl, combine cream cheese with basil leaves, oregano leaves and olives. Beat to blend well. Set aside.

Place equal amounts of the sauteed onions and mushrooms onto centers of 6 beef patties; cover each with another beef patty and pnch around edges to seal filling inside. Place burgers onto grill and cook for 4 to 5 minutes on each side. Keep warm.

To assemble burgers: spread toasted sides of buns with the cream cheese-herb spread and place equal amounts of the mixed salad greens over bottom halves of rolls, followed by one burger. Place a cheese slice atop of each burger on rolls. Last, place top half of roll atop cheese slices on each. Serve hot. Serves: 6